Compressors, for example, centrifugal compressors, operate to increase a pressure of a compressible working fluid, e.g., process gas. The process gas is received via one or more inlets at an input end of the compressor and passes through one or more impellers disposed in series on a rotatable cylindrical shaft. The shaft and the impellers may be driven by one or more motors coupled to the shaft. The pressure of the process gas increases as the process gas passes from one impeller to the next until the process gas reaches the final impeller. The compressed process gas is then expelled from the compressor via one or more outlets located at a discharge end of the compressor at a pressure greater than the pressure at which the process gas was input to the compressor.
FIG. 1 illustrates a cross-sectional view of a conventional compressor. The conventional compressor 100 includes a motor 102 (or any type of driver typically used for rotating compressors) coupled to a shaft 104. The motor 102 and/or the compressor 100 may be positioned within a housing 106. The housing 106 will generally hermetically-seal the motor 102 and the compressor 100 therein, thus providing support and protection for each component of the compressor 100. The shaft 104 may be supported at or proximate each end by at least one radial bearing, such as first and second radial bearings 108 and 110, and, depending on a length of the shaft, at one or more locations between the ends of the shaft. As shown in FIG. 1, the compressor 100 may be a multi-stage centrifugal compressor with a plurality of compressor stage impellers 112 disposed in series between the ends of the shaft 104. The compressor 100 receives process gas to be compressed from an inlet 116 (more than one inlet may be present), compresses the process gas through the successive stages of impellers 112, and thereby produces a compressed process gas. The compressed process gas exits the compressor 100 via an outlet 118 (more than one outlet may be present). The process gas, typically, includes acid gas, e.g., natural gas or any other gas mixture containing significant quantities of hydrogen sulfide (H2S), carbon dioxide (CO2), or similar acidic gases.
A balance piston 114, including an accompanying balance piston seal (not shown), may be arranged on the shaft 104 between the motor 102 and the compressor 100. The balance piston 114 is typically located behind the final impeller 112 and the backside (for example, the side of the balance piston 114 facing the motor 102 in FIG. 1) of the balance piston 114 is vented to the compressor input end via a balance line 120. As a result, any compressed process gas flowing across the balance piston seal/balance piston (also referred to as balance piston leakage) may be returned to the input end via the balance line 120 to be recompressed.
The National Association of Corrosion Engineers (NACE) Standards specify, among other things, the proper materials required to provide good service life of machinery used in acid gas environments. A NACE compliant material or component is substantially resistant to corrosion, such as the type that may occur upon exposure of a non-NACE compliant material to acid gas. Typically, materials that are exposed to the acid gas, e.g., the balance line, drillings (holes) in the compressor head and case, and other external pipes handling the acid gas, etc., are protected using NACE compliant claddings or protective sleeves.
The balance line and other external pipes that return the balance piston leakage tend to be large in size given the relatively high flow rate of the balance piston leakage passing through them and thus occupy considerable space. Also, the drillings in the compressor head typically are compound drillings (e.g., several holes in different directions) and installing claddings or protective sleeves on these compound drillings is difficult.
What is then needed is a relatively convenient method for returning the balance piston leakage to the input end of the compressor without using external plumbing or large complex drilled passages in the compressor heads or casing.